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Outcome: Biden 306 EV (D+1.2% from toss-up), Senate 50 D (D+1.0%)
Nov 3 polls: Biden 342 EV (D+5.3%), Senate 50-55 D (D+3.9%), House control D+4.6%
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How’d we do? 2010 edition

November 5th, 2010, 6:50am by Sam Wang

The near-final outcome of the 2010 elections is Senate 53D, House ~243R. What an event – the largest Republican majority in six decades. Yet a clear majority for Democrats in the Senate. This should be an interesting two years.

So how’d we do? As I wrote before, Senate poll medians did well, with Nevada as one glaring exception, leading to a 1-seat deviation from the 95% CI. There’s been talk about Hispanic voter turnout in Nevada, but I believe that’s not supported by exit polling data. It’s more likely that undecideds simply made a last-minute decision to stay with Reid.

In the House, we were off by about 13 seats, much more than the nominal error, +/-2 seats. Therefore there’s a systematic error. One possibility is bad time sampling – at the district level, polls are more spaced in time. However, we did just about as well as the famous guy, Nate Silver. In my case the tools were the Pollster database and a sheet of paper. By including national polls one could do better, not to mention other sources. Using these, a better topline (total # of seats) estimate than either of us came from Stochastic Democracy.

If you like district-level predictions, go read Stochastic Democracy. David Shor’s model that is theoretically fairly solidly based, enough to give true probabilities (for instance, House takeover 98%). His r^2 value for exact district-level vote share was much better than FiveThirtyEight. He needs to write all that up!

I have more to say about the subject of “snapshots,” “forecasts,” how they’re fundamentally the same thing, and what they’re good for. I have had interesting conversations with David (who is visiting Princeton this year) as well as correspondence with Andrew Gelman and Nate Silver. I am digesting it all, and will comment in a few days.

Tags: Politics

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